The present invention relates to a brake mechanism for a rotator that is suitable to, for example, a device for stopping rotation of a reel base which is provided to wind a tape of a videocassette on a hub or unwind the tape from the hub.
An example of a conventional brake mechanism for a rotator is disclosed in Japanese Patent Kokai Publication No. 136528/1992. When the rotator rotates in the clockwise direction, the brake mechanism for the rotator stops the rotation of the rotator by causing the rotator, brake arm, and brake pad to satisfy an automatic biting condition. On the contrary, when the rotator rotates in the counterclockwise direction, the brake mechanism stops the rotation of the rotator by causing the rotator, brake arm, and brake pad to satisfy a relieving condition.
As shown in FIG. 6, the automatic biting condition is defined as satisfying conditions that a point 3d at which the brake pad comes into contact with an outer surface 10a of a rotator 10 is located upstream in a rotating direction CW of the rotator 10 from a straight line 8 connecting a rotation center 2a of the brake arm and a rotation center 10b of the rotator 10 and that sin .alpha.&lt;.mu. cos .alpha. (.mu. denotes a friction coefficient of the brake pad, and .alpha. denotes an angle made by a normal 9 to the outer surface 10a of the rotator 10 that passes the point 3d at which the brake pad comes into contact with the outer surface 10a of the rotator 10 and a straight line that connects the point 3d at which the brake pad comes into contact with the outer surface 10a of the rotator 10 and the rotation center 2a of the brake arm). On the other hand, as shown in FIG. 7, the relieving condition is defined as satisfying a condition that the point 3d at which the brake pad comes into contact with the outer surface 10a of the rotator 10 is located downstream in the rotating direction CW of the rotator 10 from the straight line 8 that connects the rotation center 2a of the brake arm and the rotation center 10b of the rotator 10.
The brake mechanism that stops the rotation of the rotator by satisfying the automatic biting condition, as shown in FIG. 6, however, shows variations in braking torque, which may increase with changes in some factors such as a friction coefficient between the outer surface 10a of the rotator 10 and the brake pad and a distance (center-to-center distance) between the rotation center 10b of the rotator 10 and the rotation center 2a of the brake arm, as indicated by the characteristic curves shown in FIG. 8. In FIG. 8, a point "a" represents a rotation braking start point while a point "b" represents a moment at which the rotation of the rotator stops.